The Slitheen, an alien family, was brought to Earth on a ship. Some time after, it was relaunched from Earth with a fake alien pilot, actually a genetically-altered Earth pig, inside to fake a crash landing on a trajectory back to Earth. The ship flew past London buildings, and hit the side of Big Ben before landing in the Thames. 
The ship was said not to be crashed, but "parked" near 10 Downing Street, which the Slitheen had secretly infiltrated, so the Slitheen could easily escape in it.
It was part of the the Slitheen's plan to make the nations of the Earth nuke each other after the Slitheen launched the United Kingdom's nuclear arsenal, so that the Slitheen could profit off the irradiated remains as cheap starship fuel.